Depth filters (e.g., gradient-density depth filters) achieve filtration within the depth of the filter material. A common class of such filters are those that comprise a random matrix of fibers, bonded (or otherwise fixed) to form a complex, tortuous maze of flow channels. Particle separation in these filters generally results from entrapment by, or adsorption to, the fiber matrix. In gradient-density depth filters, several fiber-based filter materials (e.g., in mat or pad format) of different average nominal pore size are arranged sequentially in progressively increasing retentiveness.
Current technologies relative to depth filtration, particularly for applications requiring the removal of submicron particles, are limited to comparatively thin layer(s) of filtration material. With respect to common biopharmaceutical applications, the two most popular clarification fiber-based depth filter materials are dry-laid, gradient density, synthetic fiber pads and wet-laid, cellulose fiber sheets (with or without embedded inorganic filter aids). In either case, the overall filter material depth (or thickness) is invariably restricted to less than 10 millimeters (0.394 inches), primarily due to the high hydraulic resistance (i.e., native pressure drop per unit of fluid flow per unit area) of the filter material. Accordingly, to assure an acceptable rapid rate of fluid throughput, fiber-based depth filters based on conventional filter materials and formats are designed with an eye towards maximizing filter surface area.
The conventional predisposition towards use of large surface area filter materials is evident in the wide popularity of the saucer-like lenticular design of conventional fiber-based depthfilters for pharmaceutical and biopharmaceutical fluid clarification processes. See e.g., FIG. 7A. In practice, several of these double-sided “platters” are stacked within an external housing to effect parallel filtration (i.e., contemporaneously by each “platter”) of fluid brought into the housing. Examples of this common filter system design can be found in, for example, U.S. Pat. No. 4,783,262, issued to E. A. Ostreicher et al. on Nov. 8, 1988; and U.S. Pat. No. 5,055,192, issued to A. Artinyan et al. on Oct. 8, 1991.
In general, high-area depth filter systems are bulky, prone to poor reliability, and often have high manufacturing costs. An extensive arrangement of media seals is often required to keep the system “tight” and/or non-leaky. And, unlike surface filter media,—such as porous films or membranes which typically can be effectively packaged in a wrapped or pleated cartridge on account of their relative thinness—depth filter media do not tolerate as well the mechanical stresses typical of high surface area cartridge fabrication.
High-area depth filter systems tend also to be slow. It is well known that the filtration performance of a conventional fiber-based depth filter, as measured by filter capacity (i.e., solids loading) or particle retention or both, generally increases as fluid velocity or flux rate decreases. Particle capture—by size and amount—is more generally effective the slower particles travel through a thin filter media. (The mechanism(s) underlying particle capture are not well understood, but likely involve sieving, adsorption, and impingement.) Slow rates (per unit area) also minimize the operating pressures required which at any higher flow rate can be impractical for the high density (low permeability) filter media employed in such applications.
In light of the above, there is a need currently for a compact high-capacity deep gradient-density filter device that is reliable, robust, and easy to manufacture.